How to "ground" a character's legs?
What are you looking for is called Inverse Kinematics (IK):

In edit mode extrude a new bone from the leg (I called it "Foot On Ground") which will be the target of the IK Chain. Set it to "non deform" (uncheck the relative button in the bone properties panel) and parent it to the chest bone.
In pose mode select the leg bone and assign her an IK constraint.
In the empty boxes select "Armature" (it's the name of your armature) and "Foot On Ground". Set the IK chain lenght to 2.
Now the IK chain, 2 bones long (leg and hip Left) will always "point" to the target bone (Foot on ground).



I suggest you to copy some simple estabilished rigs before building your own, trying to understand the basic concepts. Search for Nathan Vegdahl rigging serie (free on Youtube) for an amazing advanced rigging course, from scratch to humane rigging.